Historically, there have been three general ways to produce polyimide foams. According to the process described in Lavin et al. U.S. 3,554,939, a monomer mixture composed of an ester of benzophenone tetracarboxylic acid and a polyamine, the mixture having a volatile content (defined as percent weight loss in 10 minutes at 300.degree. C.) of at least 9%, is heated to a critical temperature at which foaming occurs contemporaneously with polymerization of the tetracarboxylic and polyamine components until the polyimide foam is formed.
In another procedure, described by Gagliani in Final Report NAS 9-14718 entitled "Fire Resistant Resilient Foams" dated February 1976, a mixture of diamines is added to an alcoholic solution of the half ester of benzophenone tetracarboxylic acid and reacted at 158.degree.-167.degree. F. (70.degree.-75.degree. C.) to form a heavy syrup. This syrup is heated in a circulating air oven at 180.degree. F. (82.2.degree. C.) for about 12-16 hours and then in a vacuum oven at 176.degree.-194.degree. F. (80.degree.-90.degree. C.) for 60-90 minutes, producing a polyimide precursor. Thereafter, the polyimide precursor is pulverized into a powder which is spread over aluminum foil on an aluminum plate and heated at 600.degree. F. (315.6.degree. C.) in an oven for 30 minutes to produce the foam. In a similar procedure reported by Gagliani et al. in Final Report NAS 9-15050 entitled "Development of Fire-Resistant, Low Smoke Generating, Thermally Stable End Items for Aircraft and Spacecraft" dated June 1977, the dried precursor powder formed in about the same manner was subjected, inter alia, to multi-stage heating, in which the powder was placed in a pressure vessel positioned within an oven preheated at 450.degree. F. (232.degree. C.) and held at this temperature at a reduced pressure (19.9-9.9 inches of Hg) for 15-30 minutes. The resulting foam was then postcured at 600.degree. F. (315.6.degree. C.) for 15-30 minutes in a circulating air oven.
The third procedure involves use of microwave radiation for converting the polyimide precursor into a cellular structure which normally is then subjected to final curing in a thermal oven. In actual practice the precursor is used in the form of a powder produced by spray drying an alcoholic solution of the tetracarboxylic and diamine components. See, for example, Gagliani et al. U.S. Pat. Nos. 4,296,208; 4,305,796; 4,439,381; and 4,599,365; Final Report NAS 9-15050 (supra); Final Report NAS 9-15484 entitled "Development of Fire-Resistant, Low Smoke Generating, Thermally Stable End Items for Commercial Aircraft and Spacecraft Using a Basic Polyimide Resin" and Final Report NAS 9-16009 entitled "Formulation and Characterization of Polyimide Resilient Foams of Various Densities for Aircraft Seating Applications". In U.S. Pat. Nos. 4,305,796 and 4,439,381 it is indicated that the polyimide precursor may vary from a `liquid resin` to a spreadable, paste-like formulation, depending upon the nature and quantity of any fillers added to the resin.
One of the deficiencies in the known methods for producing polyimide foams is that the density of the final foam product cannot be controlled readily over a very wide range. This is undesirable, because a foam having a certain chemical structure is useful in some applications only if it is very dense, but it may perform very well in other applications at a much lower density. A foam of low density should be less expensive.
It is known that the foaming of polyimides is related to the volatile by-products generated in the amidization and imidization reactions. According to U.S. Pat. No. 4,900,761 small changes in the solids content of the polyimide precursor lead to large changes in the density of the resultant foam, but the exemplified changes are random and unpredictable. It is also known in the art to incorporate blowing agents of various types into the polyimide precursor, in order to decrease the density of the foam. For example, U.S. Pat. No., 4,476,254; 4,518,717 and 4,621,015 disclose the incorporation of various solid hydrates, such as oxalic acid dihydrate. The incorporation of other solid blowing agents, such as azocarbonamide and boric acid, is disclosed in U.S. Pat. No. 4,506,038. The incorporation of glass microballoons, which may increase the foam density, is described in U.S. Pat. No. 4,353,998. The incorporation of such blowing agents, however, can lead to an irregular cellular structure in the foam because of the inhomogeneity of the mixture, and residual blowing agent can adversely affect the physical properties of the resultant foam.